Conventional electrochemical cells are designed and manufactured to use ampere-hour electrode capacities within a specified extent of reaction. This restriction is necessary in order to hold the cell at a specified nominal voltage. Conversely, cell voltage may be regulated within a given range so as to maintain the needed extent of reaction. For example, nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH), nickel-hydrogen (Ni-H.sub.2), and lead acid chemistries have average nominal cell voltages of 1.2V, 1.23V, 1.23V, and 2.1V, respectively.
Apparatus and methods for charging these types of batteries typically rely upon a single change in the slope of the battery voltage charging curve in order to determine when charging should be terminated. For example, U.S. Pat. No. 4,639,655 to Westhaver, et. al., for "METHOD AND APPARATUS FOR BATTERY CHARGING" discusses a charging regime which looks for a "knee region" in the battery charging curve to determine the stop point in the charging regime. The "knee region" is that part of the charging curve where the slope of the curve (i.e., the rate of change of cell voltage versus time) either begins to decrease, or in fact becomes negative.
Similarly, U.S. Pat. No. 4,388,582 to Saar, et. al. for "APPARATUS AND METHOD FOR CHARGING BATTERIES" discusses monitoring a battery characteristic, such as battery voltage, to identify "inflection points" in the charging curve. These inflections points are related to changes in the slope of the charging curve, and indicate at what stage, and when the charge should be terminated. The charge is typically terminated when the slope of the curve either decreases, or becomes negative.
These battery charging regimes will not however, work with the multiple voltage battery cell disclosed and claimed in the referenced '812 application. Since the '812 cell is multiple voltage, (i.e., operates at two or more distinct operational voltage regimes) the cell is characterized by a charging curve wherein the slope of the charging voltage versus time, (for example) decreases at least twice. Using the methods described in the above-referenced patents, the multiple voltage cell described in the '812 application would be only partially charged at best.
Accordingly, there exists a need to provide a cell charging apparatus, and charging regime capable of fully charging multiple voltage electrochemical cells capable of operating at multiple voltage levels.